1. Field of the Invention
This invention relates broadly to data communication systems for downhole devices. More particularly, this invention relates to data communication systems over a power cable to an electrical submersible pump (ESP).
2. Description of Related Art
Various communication systems for telemetry signals that represent physical parameters measured by sensors within a borehole have been proposed and/or utilized. Several such systems employ the three-phase power cable (which transmits power to the ESP) for transmission of such telemetry signals.
An example of such a system is shown in U.S. Pat. No. 5,515,038. In this system, direct current (DC) offsets to the alternating current (AC) power signal carried on the cable are used to carry the telemetry data.
Disadvantageously, such communication systems cannot tolerate a ground fault, for example, where one conductor of the power cable is shorted to ground. Moreover, such systems typically employ large isolation chokes on the surface and in the downhole environment, which limit the data transfer rates of the system, and also fail to operate in noisy environments.
In another system shown in U.S. Pat. No. 6,587,037, phase-to-phase signaling is used to carry the telemetry data over the conductors of the power cable. Disadvantageously, the communication requires access to all three conductors in the downhole environment, which is difficult and expensive to implement and maintain.
Thus, there is a need in the art to provide a communication system for downhole devices that is tolerant to ground faults and is also cost-effective. Moreover, the communication system should advantageously avoid the use of large isolation chokes, provide relatively high data transfer rates, provide two-way communication, and also operate efficiently in noisy environments.